In an optical network system, in order to specify a location where a communication failure occurs, a command is sequentially transmitted from an optical transmission device to each of relay devices one by one on a transmission line, and a relay device or a transmission section where the communication failure occurs is specified depending on a presence or a non-presence of a response to the command.
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-032192 and Japanese Laid-Open Patent Publication No. 2002-280978.